U.S. patent number 7,993,373 [Application Number 11/063,941] was granted by the patent office on 2011-08-09 for polyaxial orthopedic fastening apparatus.
Invention is credited to T. Wade Fallin, Robert W. Hoy, Daniel F. Justin, David W. Meibos.
United States Patent |
7,993,373 |
Hoy , et al. |
August 9, 2011 |
Polyaxial orthopedic fastening apparatus
Abstract
An orthopedic fastener is designed to attach an implant to bone
in such a manner that, until the fastener is tightened, the implant
may be rotationally adjusted against the bone. The implant may have
a semispherical bone apposition surface that permits polyaxial
rotation of the implant against the bone. The orthopedic fastener
has an interpositional member and a compression member. The
interpositional member may be a split ring with a conical exterior
surface that mates with a conical surface of the implant. The
compression member has a threaded bore that engages a proximal end
of a fixation member implanted in the bone such that, in response
to rotation of the compression member, the interpositional member
is sandwiched securely between the implant and the compression
member. The conical surface of the implant compresses the
interpositional member about a semispherical surface of the
compression member, thereby restricting rotation of the
implant.
Inventors: |
Hoy; Robert W. (Paradise,
UT), Fallin; T. Wade (Hyde Park, UT), Justin; Daniel
F. (Logan, UT), Meibos; David W. (Draper, UT) |
Family
ID: |
36927931 |
Appl.
No.: |
11/063,941 |
Filed: |
February 22, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060200149 A1 |
Sep 7, 2006 |
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Current U.S.
Class: |
606/247; 606/308;
606/310; 606/306 |
Current CPC
Class: |
A61B
17/7064 (20130101); A61F 2/4405 (20130101); A61B
17/70 (20130101); A61F 2002/30405 (20130101); A61F
2002/3085 (20130101); A61F 2250/0006 (20130101); A61F
2220/0041 (20130101); A61F 2002/30485 (20130101); A61F
2220/0025 (20130101); A61F 2002/30016 (20130101); A61F
2002/30433 (20130101); A61F 2250/0019 (20130101); A61F
2002/30507 (20130101); A61F 2002/4677 (20130101); A61F
2002/30538 (20130101) |
Current International
Class: |
A61B
17/70 (20060101) |
Field of
Search: |
;606/72,73,60,246-279,300-331 ;623/17.11-17.16 ;411/380,401,396
;403/374.3,374.4 |
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Primary Examiner: Robert; Eduardo C
Assistant Examiner: Cumberledge; Jerry
Claims
The invention claimed is:
1. An apparatus comprising: a prosthesis comprising a bone
contacting surface comprising a three-dimensional section of a
sphere and an articular surface having a generally planar surface
shaped to replace a natural articular surface of a bone; and an
orthopedic fastener configured to secure the prosthesis to a bone,
the orthopedic fastener comprising: a compression member comprising
a bore defining a first center axis; and an interpositional member
comprising a split ring comprising a flat circular end comprising a
center point, and a second center axis extending perpendicular to
the flat circular end and through the center point, wherein the
interpositional member cooperates with the compression member to
provide an unlocked configuration in which the interpositional
member is rotatable with respect to the compression member such
that the first axis is capable of being selectively oriented to
parallel and nonparallel positions relative to the second axis, and
a locked configuration in which the second axis is maintained in
its selected parallel or nonparallel orientation relative to the
first axis and relative rotation between the interpositional member
and the compression member is restricted; wherein the orthopedic
fastener is configured to move from the unlocked configuration to
the locked configuration in response to urging of the compression
member toward the bone to hold the three-dimensional section
against the bone.
2. The apparatus of claim 1, further comprising a fixation member
implantable in the bone, the fixation member having a receiving
interface configured to cooperate with the compression member to
urge the compression member toward the bone.
3. The apparatus of claim 2, wherein the fixation member comprises
a distal end shaped to be implanted in the bone, and a proximal end
having threads, wherein the compression member comprises threads
shaped to cooperate with the threads of the proximal end to cause
the compression member to advance toward the distal end in response
to rotation of the compression member with respect to the fixation
member.
4. The apparatus of claim 1, wherein the bone comprises a
vertebra.
5. The apparatus of claim 4, wherein the articular surface is
shaped to replace a natural articular surface of a facet of the
vertebra.
6. The apparatus of claim 1, wherein the bone contacting surface
comprises a generally semispherical shape.
7. The apparatus of claim 1, wherein at least one of the
compression member and the interpositional member is configured to
deform in response to motion of the compression member toward the
bone to lock against the other of the compression member and the
interpositional member.
8. The apparatus of claim 7, wherein at least one of the
compression member and the interpositional member comprises a
plurality of features that protrude toward the other of the
compression member and the interpositional member to restrict
relative rotation between the compression member and the
interpositional member.
9. The apparatus of claim 7, wherein the interpositional member
comprises a split ring configured to contract in response to motion
of the compression member toward the bone.
10. The apparatus of claim 1, wherein the interpositional member
comprises a prosthesis interface having a generally conical shape,
wherein the prosthesis comprises an interpositional interface
having a generally conical shape that mates with the prosthesis
interface to restrict rotation of the prosthesis with respect to
the bone about the second axis.
11. The apparatus of claim 10, wherein at least one of the
prosthesis interface and the interpositional interface comprises a
plurality of features that protrude toward the other of the
prosthesis interface and the interpositional interface to restrict
relative rotation between the prosthesis and the interpositional
member.
12. An apparatus comprising: a prosthesis comprising an articular
surface shaped to replace a natural articular surface of a facet of
a vertebra and configured and dimensioned to articulate with a
natural articular surface of an adjacent facet; an orthopedic
fastener configured to secure the prosthesis to the vertebra, the
orthopedic fastener comprising: a compression member; and an
interpositional member comprising a locking surface, a prosthesis
interface, a flat circular end extending between the locking
surface and the prosthesis interface and comprising a center point,
and a center axis, wherein the center axis is perpendicular to the
flat circular end and passes through the center point, wherein the
locking surface engages the compression member to provide an
unlocked configuration in which the interpositional member is
rotatable with respect to the compression member, and a locked
configuration in which relative rotation between the
interpositional member and the compression member is restricted,
and the prosthesis interface is shaped to engage the prosthesis to
press the prosthesis toward the vertebra in response to urging of
the compression member toward the vertebra; and a fixation member
implantable in the vertebra, wherein the fixation member is adapted
and configured to be selectively oriented to parallel and
nonparallel orientations relative to the center axis in the
unlocked configuration, and the fixation member is selectively
fixed in a parallel or nonparallel orientation relative to the
center axis in the locked configuration, the fixation member having
a receiving interface configured to cooperate with the compression
member to urge the compression member toward the vertebra; wherein
the orthopedic fastener is configured to move from the unlocked
configuration to the locked configuration in response to urging of
the compression member toward the vertebra.
13. The apparatus of claim 12, wherein the fixation member
comprises a distal end shaped to be implanted in the vertebra, and
a proximal end having threads, wherein the compression member
comprises threads shaped to cooperate with the threads of the
proximal end to cause the compression member to advance toward the
distal end in response to rotation of the compression member with
respect to the fixation member.
14. The apparatus of claim 12, wherein at least one of the
compression member and the interpositional member is configured to
deform in response to motion of the compression member toward the
vertebra to lock against the other of the compression member and
the interpositional member.
15. The apparatus of claim 14, wherein at least one of the
compression member and the interpositional member comprises a
plurality of features that protrude toward the other of the
compression member and the interpositional member to restrict
relative rotation between the compression member and the
interpositional member.
16. The apparatus of claim 12, wherein the interpositional member
comprises an prosthesis interface having a generally conical shape,
wherein the prosthesis comprises an interpositional interface
having a generally conical shape that mates with the prosthesis
interface to restrict rotation of the prosthesis with respect to
the vertebra about the axis.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
The following disclosure is incorporated herein by reference: U.S.
application Ser. No. 10/860,778 filed Jun. 2, 2004 which is
entitled SPINAL FACET IMPLANT WITH SPHERICAL IMPLANT APPOSITION
SURFACE AND BONE BED AND METHODS OF USE.
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates generally to systems and methods for
attaching implants to bone, and more specifically, to a polyaxial
orthopedic fastening apparatus particularly useful in the field of
facet joint replacement.
2. The Relevant Technology
Orthopedic medicine provides a wide array of implants that can be
attached to bone to alleviate various pathologies. One unique
challenge in orthopedics is to provide implants and fastening
devices that are adaptable to a variety of bone morphologies. Each
patient will have a different bone structure; accordingly, it may
be necessary to allow for adjustable positioning of an implant with
respect to the bone so that the implant will be positioned to
perform its function.
For this reason, a number of fixation systems have been invented
that enable variation of the angle between the implant and the
fastener. Although such fixation systems generally permit
adaptation to the bone morphology of a patient to provide secure
anchoring of the implant to bone, they are generally somewhat
limited in the types of adjustment they permit with respect to the
bone. Accordingly, such fixation systems may not be usable with a
number of implants that require more comprehensive adjustability.
Furthermore, many known implant fixation systems are complex due to
the presence of several parts, or due to the need to perform
several steps to utilize them to attach an implant to bone. Yet
further, some known implant fixation systems are expensive, and
require the use of unusual tooling. A need exists in the art for
implant fixation systems and methods that alleviate the foregoing
shortcomings.
BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of the present invention will now be discussed
with reference to the appended drawings. It is appreciated that
these drawings depict only typical embodiments of the invention and
are therefore not to be considered limiting of its scope.
FIG. 1 is an exploded, perspective view of a vertebra with an
apparatus according to one embodiment of the invention, with the
apparatus positioned for attachment to the vertebra.
FIG. 2 is a caudal, section view of the vertebra and the apparatus
of FIG. 1, with the orthopedic fastener of the apparatus in the
unlocked configuration to permit adjustment of the orientation of
the implant against the bone.
FIG. 3 is a caudal, section view of the vertebra and apparatus of
FIG. 1, with the orthopedic fastener in the locked configuration to
restrict rotational motion of the implant against the bone.
FIG. 4 is an exploded, perspective view of a vertebra with an
apparatus according to one alternative embodiment of the invention,
with the apparatus positioned for attachment to the vertebra.
DETAILED DESCRIPTION
The present invention advances the state of the art by providing
systems and methods that can be used to anchor orthopedic implants
to bone in a manner that provides a high degree of implant
adjustability, simplicity, and ease of use. The present invention
can be used in any orthopedic procedure, but may have particular
utility in the field of facet joint replacement to alleviate back
pain resulting from traumatic, inflammatory, metabolic, synovial,
neoplastic and degenerative spinal disorders. The configuration and
operation of selected embodiments of the invention will be shown
and described in greater detail with reference to FIGS. 1 through
4, as follows.
In this application, the terms "compression member" and
"interpositional member" are used broadly. A "compression member"
generally is a member that receives a compressive force. An
"interpositional member" generally is a member, at least part of
which is designed to be positioned between at least two other
members of a system. Deformation of one part "to lock" another part
generally relates to deformation of the first part in such a manner
that the first part physically impedes relative motion between the
two parts, rather than simply supplying frictional force to provide
locking. However, the present invention encompasses embodiments
that provide locking solely via one or more frictional forces.
"Polyaxial" rotation is rotation that can occur about at least two
axes that are not parallel to each other. "Triaxial rotation" is
rotation about three perpendicular axes. Triaxial rotation is
equivalent to rotation about a point, because free rotation about
any axis of a 3D coordinate system is the same as rotation that is
not limited to any axis in the system.
Referring to FIG. 1, a perspective view illustrates an apparatus 10
according to one embodiment of the invention, in use with a
vertebra 12, such as an L5 lumbar vertebra of a human spine. As
shown, the vertebra 12 has a body 18, which is generally
disc-shaped. The vertebra 12 also has two pedicles 20 extending
from the body 18, and a posterior arch, or lamina 22, that extends
between the posterior ends of the pedicles 20 to couple the
pedicles 20 together. The vertebra 12 also has a pair of transverse
processes 24 that extend laterally from the pedicles 20, and a
spinous process 26 that extends posteriorly from the lamina 22.
The vertebra 12 also has a pair of superior facets 28, which are
positioned toward the top of the vertebra 12 and face generally
medially. Additionally, the vertebra 12 has inferior facets 30,
which are positioned toward the bottom of the vertebra 12 and face
generally laterally. Each of the pedicles 20 of the vertebra 12 has
a saddle point 32, which is positioned generally at the center of
the juncture of each superior facet 28 with the adjacent transverse
process 24.
The superior facets 28 of the vertebra 12 articulate (i.e., slide
and/or press) against the inferior facets (not shown) of an
adjacent superior vertebra (not shown) to limit relative motion
between the vertebra 12 and the superior vertebra. Thus, the
combination of each superior facet 28 with the adjacent inferior
facet defines a facet joint (not shown). Accordingly, two facet
joints span the distance between each adjacent pair of vertebrae.
The inferior facets 30 of the vertebra 30 are part of other facet
joints that control motion between the vertebra 12 and all adjacent
inferior vertebra (not shown) and/or the sacrum (also not
shown).
Each of the facet joints may be covered by a capsule (not shown)
containing a fluid (not shown) that reduces wear of the facets 28,
30 and facilitates articulation. Additionally, layers of cartilage
(not shown) may cover the facets 28, 30 to further reduce wear and
facilitate articulation. These anatomical structures, as well as
the various muscles, ligaments, and nerves of the spine, will not
be depicted in the Figures to enhance the clarity of the
disclosure. Such structures may be removed or displaced according
to known methods to provide the necessary access to the vertebra
12.
As shown, a semispherical resection 34 has been formed on one of
the saddle points 32 of the vertebra 12. The semispherical
resection 34 is shaped to receive an implant to replace the
articular surface of one or both of the adjacent superior and
inferior facets 28, 30. The semispherical resection 34 permits
relative rotation between the implant and the vertebra 12 about
three perpendicular axes prior to fixation of the implant to the
vertebra 12. The axes may be defined as shown by reference numerals
40, 42, and 44 in FIG. 1.
More precisely, the axes may include a first axis 40, a second axis
42, and a third axis 44. The first axis 40 is generally collinear
with the axis of the corresponding pedicle 20. The second axis 42
is generally vertical (i.e., parallel to the axis of the body 18)
and perpendicular to the first axis 40. The third axis 44 is
generally horizontal (i.e., parallel to the end plates of the body
18) and perpendicular to the first and second axes 40, 42.
The apparatus 10 includes an implant 50, a fixation member 52, and
an orthopedic fastener 54, or fastener 54. The implant 50 is
designed to seat against the semispherical resection 34 and to
replace the articular surface of the inferior facet 30 immediately
inferior to it. The fixation member 52 may take the form of a
pedicle screw designed to be implanted in the corresponding pedicle
20 to anchor the implant 50 in place. The orthopedic fastener 54 is
designed to be coupled to the fixation member 52 to hold the
implant 50 against the vertebral 12. In FIG. 1, the components of
apparatus 10 are shown aligned on a longitudinal center axis 46 of
the apparatus 10.
In the embodiment of FIG. 1, the implant 50 has a fixation portion
60, an articulation portion 62, and a stem 64. The fixation portion
60 is shaped to be attached to the semispherical resection 34, and
the articulation portion 62 provides a surface that articulates
with an adjacent vertebral facet to carry out the function of the
inferior facet 30. The articulation portion 62 is coupled to the
fixation portion 60 by the stem 64.
As shown, the fixation portion 60 has a bone apposition surface 66,
which may be generally semispherical to correspond to the shape of
the semispherical resection 34. The fixation portion 60 also has an
aperture (not visible in FIG. 1) that passes through the bone
apposition surface 66 to receive the fixation member 52. The
aperture is somewhat larger than the exterior surface of the
fixation member 52 so that the bone apposition surface 66 is able
to slide against the semispherical resection 34 with the fixation
member 52 in place, implanted in the pedicle 20. It can be seen
that the fixation portion 60 has a flat proximal end, shown facing
the fastener 54. The flat proximal end has a circular inner edge,
which inherently has a center point. A center axis of the fixation
portion 60 may be defined through the center of the circular inner
edge and perpendicular to the flat proximal end. In FIG. 1, the
center axis of the fixation portion 60 is collinear with axis
46.
The articulation portion 62 similarly has an articulation surface
68 designed to articulate with a superior facet of a vertebra
immediately inferior to the vertebra 12. The articulation surface
68 may have a convex shape, which may further be semispherical,
semicylindrical, or the like. The articulation surface 68 may be
designed to articulate with a natural superior facet or a
prosthetic superior facet.
In addition to the bone apposition surface, the fixation portion 60
also has an interpositional interface 70 shaped to interact with
the fastener 54 in a manner that will be described subsequently.
The interpositional interface 70 has a generally conical surface
that converges to the aperture of the fixation portion 60.
In the embodiment of FIG. 1, the fixation member 52 has a distal
end 74 implanted into one of the pedicles 20 of the vertebra 12,
and a proximal end 76 that protrudes from the corresponding saddle
point 32. The distal end 74 has threads (not visible in FIG. 1)
that facilitate implantation of the distal end 74 in the pedicle 20
and keep the implanted distal end 74 in place. The proximal end 76
has a plurality of threads 78 that are exposed to receive the
fastener 54. Additionally, the proximal end 76 has a torquing
interface that may be used to apply torque to the fixation member
52 to implant the distal end 74 in the pedicle 20. The torquing
interface may take the form of a hexagonal recess into which a
hexagonal driver end can be inserted. The distal threads, proximal
threads, and torquing interface are aligned on a longitudinal
center axis of the fixation member 52. In FIG. 1, the longitudinal
center axis of the fixation member 52 is collinear with axis
46.
As shown in FIG. 1, the fastener 54 includes an interpositional
member 82 and a compression member 84. The interpositional member
82 may take the form of a split ring, as illustrated. It can be
seen that the split ring has a flat proximal end, shown facing the
compression member 84. The flat proximal end has a circular outer
edge, which inherently has a center point. A center axis of the
split ring may be defined through the center point of the circular
outer edge and perpendicular to the flat proximal end. In FIG. 1,
the center axis of the split ring is collinear with axis 46. The
compression member 84 is designed to be advanced along the proximal
end 76 of the fixation member 52 to press the interpositional
member 82 into the interpositional interface 70 of the implant
50.
More specifically, the interpositional member 82 has an implant
interface 88, a compression interface 90, and a gap 92. As embodied
in FIG. 1, the implant interface 88 includes a generally conical
exterior surface designed to mate with the interpositional
interface 70 in such a manner that the interpositional member 82 is
compressed inward relatively uniformly as it is urged into the
interpositional interface 70. The compression interface 90 includes
a generally semispherical interior surface designed to receive a
corresponding surface of the compression member 84. The gap 92
enables the interpositional member 82 to obtain a relatively high
degree of deflection to provide two very distinct configurations:
an unlocked configuration, in which the interpositional member 82
is relatively undeflected, and a locked configuration in which the
interpositional member 82 is compressed to narrow or remove the gap
92.
The interpositional member 82 may be formed of a material with a
relatively low stiffness, such as plastic or rubber. A low
stiffness provides relatively high deflection in the
interpositional member 82 without requiring excessive force.
Accordingly, moving the fastener 54 between the unlocked and locked
configurations is relatively easily accomplished.
The compression member 84 has an interpositional interface 96, a
torquing interface 98, and a bore 100. The interpositional
interface 96 is shaped to mate with the compression interface 90 of
the interpositional member 82. More specifically, the
interpositional interface 96 may include an exterior, semispherical
surface positionable within the compression interface 90. The
torquing interface 98 may comprise a castle nut interface, with a
plurality of radial projections that can be engaged by the end of a
tool (not shown) with projections that mesh with the torquing
interface 98. The torquing interface 98 is able to receive torque
from such a tool to enable the compression member 84 to be advanced
along the proximal end 76 of the fixation member 52.
The bore 100 passes through the compression member 84 and is sized
to receive the proximal end 76. The bore 100 has threads (not
shown) that engage the threads 78 of the proximal end 76 such that
the compression member 84 advances along the proximal end 76 in
response to rotation of the compression member 84. The threads in
the bore 100 and the torquing interface 98 are aligned on a
longitudinal center axis of the compression member 84. In FIG. 1,
the longitudinal center axis of the compression member 84 is
collinear with axis 46.
The compression interface 90 and the interpositional interface 96
are sized such that they mate together with clearance when the
interpositional member 82 is relatively undeflected. Thus, the
interpositional member 82 and the compression member 84 are able to
rotate with respect to each other about all three axes 40, 42, 44.
Accordingly, when the interpositional member 82 is relatively
undeflected or less deflected, the fastener 54 is in an unlocked
configuration. When the interpositional member 82 is compressed,
the compression interface 90 tightly engages the interpositional
interface so that relative rotation about all three axes 40, 42, 44
is restricted. Thus, when the interpositional member 82 is
relatively compressed, the fastener 54 is in a locked
configuration.
The interpositional interface 96 may have features, such as a
plurality of ridges 102, that are designed to engage the
interpositional member 82 in the locked configuration to enhance
locking. The ridges 102 extend around the circumference of the
interpositional interface 96, about the first axis 40. Each of the
ridges 102 has a faceted shape, as shown in FIG. 1, so that each
ridge 102 has multiple relatively sharp projections that extend
outward from the generally semispherical shape of the
interpositional interface 96.
The compression member 84 may be formed of a material harder than
that of the interpositional member 82. For example, the compression
member 84 may be formed of a biocompatible metal. Accordingly, the
projections of the ridges 102 may embed themselves into the
compression interface 90 when the fastener 54 is moved to the
locked configuration to enhance locking by resisting relative
rotation between the interpositional member 82 and the compression
member 84.
The interpositional interface 96 and the compression interface 90
may be relatively sized such that, in the unlocked configuration,
there is little enough clearance that the interpositional member 82
and the compression member 84 will generally remain assembled.
Thus, the interpositional member 82 and the compression member 84
may be pre-assembled (i.e., factory assembled or the like), so that
the fastener 54 is ready for use at the commencement of the
surgical procedure without further assembly.
The fastener 54 may be used in concert with the fixation member 52
to retain the implant 50 in any of a plurality of orientations with
respect to the vertebra 12. The manner in which this is carried out
will be shown and described in greater detail with reference to
FIGS. 2 and 3, as follows.
Referring to FIG. 2, a cephalad, section view illustrates the
apparatus 10 and the vertebra 12, with the apparatus 10 fully
assembled and with the fastener 54 in the unlocked configuration.
As shown, the distal end 74 of the fixation member 52 has threads
104 that are embedded in the body 18 and in the corresponding
pedicle 20 of the vertebra 12. The fixation portion 60 of the
implant 50 has an aperture 106 to which the generally conical
surface of the interpositional interface 70 of the implant 50
converges. The bore 100 of the compression member 84 has threads
108 that engage the threads 78 of the proximal end 76 of the
fixation member 52. These elements were briefly described in the
discussion of FIG. 1, but were not visible in FIG. 1.
In order to install the implant 50, the posterior elements of the
vertebra 12 may first be exposed through the use of procedures
known in the art. Measurements may be made to select the implant 50
and determine the appropriate orientation for the implant 50, and
any other implants to be installed in the same operation. The
semispherical resection 34 may be formed via a reaming operation or
the like, and other portions of the vertebra 12, such as the
inferior facet 30 to be replaced, may be resected as needed. The
fixation member 52 may be implanted in the corresponding pedicle 20
along the desired angle.
The implant 50 may then be inserted and positioned such that the
bone apposition surface 66 abuts the semispherical resection 34.
Prior to adjustment of the orientation of the implant 50, the
fastener 54 may be installed so that it can be used to easily fix
the implant 50 in place once it has reached the desired
orientation. As mentioned previously, the interpositional member 82
and the compression member 84 may be pre-assembled, and therefore,
they may not need to be assembled prior to implantation.
The compression member 84, with the interpositional member 82
already coupled to it, may be positioned so that the proximal end
76 of the fixation member 52 passes into the bore 100 of the
compression member 84. The compression member 84 may then be
rotated to cause the threads 108 of the bore 100 to engage the
threads 78 of the proximal end 76. The interpositional member 82 is
inserted into the interpositional interface 70 of the implant 50
such that the generally conical implant interface 88 of the
interpositional member 82 engages the generally conical surface of
the interpositional interface 70. The interpositional member 82 is
thereby drawn into coaxiality with the fixation portion 60 of the
implant 50.
Further rotation of the compression member 84 will urge the
interpositional member 82 further along the interpositional
interface 70, toward the pedicle 20. The engagement of the
generally conical surfaces of the interpositional interface 70 and
the implant interface 88 causes the interpositional member 82 to
contract in response to such motion, thereby bringing the fastener
54 to the locked configuration. Until the implant 50 has been
adjusted to the desired orientation, the compression member 84 may
be rotated just far enough to keep it in place to facilitate
subsequent tightening, but not far enough to move the fastener 54
to the locked configuration.
Thus, the apparatus 10 reaches the configuration shown in FIG. 2.
The fastener 54 is coupled to the proximal end 76 of the fixation
member 52, but has not been tightened. The center axis of the
compression member 84 and the center axis of the fixation member 52
remain collinear with axis 46. Accordingly, although the
orientation of the interpositional member 82 is substantially fixed
with respect to the implant 50, so that the center axis of the
fixation portion 60 and the axis of the interpositional member 82
are collinear, the interpositional member 82 and implant 50 are
rotatable about any of the axes 40, 42, 44 with respect to the
compression member 84. Accordingly, the implant 50 is still
relatively freely rotatable about the axes 40, 42, 44 with respect
to the vertebra 12. FIG. 2 shows the components of apparatus 10
oriented so that the interpositional member 82 and implant 50 are
tilted. In this orientation, the axes of the interpositional
interface 70 and the implant interface 88 are nonparallel relative
to axis 46.
The implant 50 may then be adjusted by rotating the implant 50 with
respect to the vertebra 12 such that the bone apposition surface 66
rotates about any or all of the axes 40, 42, 44 within the
semispherical resection 34, until the articulation surface 68 is at
the desired position and angle. Then, it is desirable to fix the
orientation of the implant 50 with respect to the bone, as will be
shown and described in connection with FIG. 3.
Referring to FIG. 3, a cephalad, section view illustrates the
apparatus 10 and the vertebra 12, with the fastener 54 in the
locked configuration. After rotational adjustment of the implant 50
with respect to the vertebra 12, the compression member 84 is
further rotated, for example, by engaging the torquing interface 98
with a suitable tool, as described previously, and applying torque.
The compression member 84 is advanced along the proximal end 76,
toward the pedicle 20 so that the interpositional member 82 is
compressed by engagement with the interpositional interface 70 of
the implant 50.
As the interpositional member 82 is compressed, the gap 92 shrinks
and the compression interface 90 of the interpositional member 82
contracts to grip the interpositional interface 96. The ridges 102,
or just the projections thereof, embed themselves in the
compression interface 90 to restrict and/or entirely prevent
further rotation of the compression member 84 with respect to the
interpositional member 82. The interpositional interface 70 of the
implant 50 and the implant interface 88 of the interpositional
member 82 engage each other in such a manner that relative rotation
between the implant 50 and the interpositional member 82 is
substantially prevented about the second and third axes 42, 44.
Relative rotation about the first axis 40 is also restricted as the
interpositional member 82 is compressed, and therefore, provides
frictional force as it presses outward against the interpositional
interface 70.
Accordingly, when the compression member 84 is advanced along the
proximal end 76 to provide the locked configuration, relative
rotation between the vertebra 12 and the implant 50 are
substantially prevented. This is the configuration illustrated in
FIG. 3. The implant 50 is locked in its preferred orientation with
respect to the vertebra 12. The implant 50 can be fixed in any of a
wide variety of different orientations with respect to the vertebra
12 through the use of the fixation member 52 and the fastener
54.
The embodiment shown in FIGS. 1, 2, and 3 is only one of many
embodiments of the present invention. In alternative embodiments,
differently shaped implants, interpositional members, compression
members, and fixation members may be used. For example, generally
conical surfaces need not be used to interface between an implant
and an interpositional member. Such an interfacing surface may
include cylindrical, semispherical, conical, parabolic, or other
shapes, or combinations thereof. According to one alternative
embodiment, an interfacing surface may include a cylindrical
component adjoining a semicylindrical or parabolic component that
flares the interface to provide a diameter larger than that of the
cylindrical component. Such an interfacing surface may also have a
polygonal cross section, or may be keyed or otherwise shaped to
limit an implant to a discrete number of relative orientations with
respect to a vertebra.
Similarly, generally semispherical surfaces need not be used to
interface between the interpositional member and the compression
member. Such interfacing surfaces may include cylindrical,
semispherical, conical, parabolic, or other shapes, or combinations
thereof. By contrast with the embodiment of FIGS. 1, 2, and 3,
alternative embodiments of the invention need not necessarily
provide rotation of an implant with respect to a vertebra about
three perpendicular axes.
In some alternative embodiments, additional features may be added
to enhance locking provided by the locked configuration. Such
features may enable an implant to receive greater loads without
moving from its preferred orientation with respect to the vertebra.
One example of such an alternative embodiment will be shown and
described in connection with FIG. 4, as follows.
Referring to FIG. 4, a perspective view illustrates an apparatus
110 according to one alternative embodiment of the invention, used
in conjunction with a vertebra 12, as described previously. As
shown, the apparatus 110 includes an implant 150, a fixation member
52, and an orthopedic fastener 154, or fastener 154. The fixation
member 52 may be identical to that of the previous embodiment. The
implant 150 and the fastener 154 are similar to their counterparts
of previous embodiment, but include additional features to enhance
locking, as will be described below.
As shown, the implant 150 has a fixation portion 160 designed to be
attached to the semispherical resection 34. The fixation portion
160 has an interpositional interface 170, which has a generally
conical surface like that of the interpositional interface 70 of
the previous embodiment. However, the interpositional interface 170
does not provide a smooth conical surface. Rather, the
interpositional interface 170 has a plurality of ridges 172
distributed along a generally radially symmetrical pattern about
the generally conical surface.
The ridges 172 protrude inward to resist relative rotation between
the implant 150 and an interpositional member 182 of the fastener
154. Like the interpositional member 82 of the previous embodiment,
the interpositional member 182 has an implant interface 88 with a
generally conical shape. The implant interface mates with the
interpositional interface 170 of the implant 150. By contrast with
the previous embodiment, the ridges 172 of the interpositional
interface 170 are able to penetrate the generally conical surface
of the implant interface 88 to restrict, or even substantially
prevent, relative rotation between the implant 150 and the
interpositional member 182 about the first axis 40.
Penetration of the implant interface 88 by the ridges 172 may occur
because the implant 150 may be formed of a material that is harder
than that of the interpositional member 182. For example, the
interpositional member 182 may be formed of a polymer, elastomer,
or the like, while the implant 150 may be formed of a metal, a
ceramic, or a harder polymer or elastomer. Thus, the ridges 172 may
serve to enhance the locking provided by the locked configuration
of the fastener 154.
In addition to the interpositional member 182, the fastener 154 has
a compression member 84, which is identical to that of the previous
embodiment. However, the interpositional member 182 differs from
the interpositional member 82 of the previous embodiment in that
the interpositional member 182 has a compression interface 190 with
features designed to enhance locking with the interpositional
interface 96 of the compression member 84. More precisely, the
compression interface 190 includes a generally semispherical
surface like that of the previous embodiment. However, the
compression interface 190 has a plurality of ridges 192 that extend
along generally circular pathways about the circumference of the of
the compression interface 190.
In the locked configuration, the ridges 192 enhance locking by
helping prevent rotation between the compression interface 190 and
the interpositional interface 96 of the compression member 84. More
precisely, the ridges 192 may deform against the interpositional
interface 96 of the compression member 84, thereby providing
regions in which the frictional force between the compression
interface 190 and the interpositional interface 96 is relatively
large. The ridges 192 may also directly block motion of the ridges
102 of the interpositional interface 96, thereby particularly
restricting relative rotation between the interpositional member
182 and the compression member 84 about the second and third axes
42, 44.
Thus, the ridges 172 and the ridges 192 that have been added in the
apparatus 110 of FIG. 4 may serve to enhance locking of the
orientation of the implant 150 with respect to the vertebra 12, in
the locked configuration of the fastener 154. Those of skill in the
art will recognize that alternative features may be added to
enhance locking. In selected alternatives, the ridges 172 of the
interpositional interface 170 of the implant 150 may follow
non-converging or curvilinear pathways designed to avoid
interfering with compression of the interpositional member 182.
Furthermore, the differently-shaped surfaces described previously
for interfacing between the implant 50 and the interpositional
member 82 or between the interpositional member 82 and the
compression member 84 may be used in conjunction with features like
the ridges 172 and the ridges 192 of the embodiment of FIG. 4 to
provide enhanced locking for such alternative interfacing surface
shapes.
The present invention has particular relevance to orthopedic
medicine, and more particularly to facet joint replacement.
However, the principles, structures, and methods of the present
invention may also be extended to a wide variety of other
fields.
The present invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. As
such the described embodiments are to be considered in all respects
only as illustrative and not restrictive. The scope of the
invention is, therefore, indicated by the appended claims rather
than by the foregoing description. All changes which come within
the meaning and range of equivalency of the claims are to be
embraced within their scope.
* * * * *
References